Synthetic spectra of energetic core-collapse supernovae and the early spectra of SN 2007bi and SN 1999as

SN 2007bi and SN 1999as are among the first superluminous supernovae discovered. SN 2007bi was suggested to be powered by the radioactive decay of a large amount (5-10 M) of ⁵⁶Ni. SN 1999as has a similar spectrum to SN 2007bi. One suggested way to synthesize such a large amount of ⁵⁶Ni is through energetic core-collapse supernovae from very massive progenitors. Although the synthetic light curves of extremely energetic core-collapse supernovae have been shown to be consistent with SN 2007bi, no synthetic spectra have been reported. Here, we present synthetic spectra of extremely energetic core-collapse supernovae during the photospheric phases. We find that the ejecta density structure above 13 000-16 000 km s⁻¹ needs to be cut in order to explain the coexisting broad- and narrow-line absorptions in SN 2007bi and SN 1999as. The density cut is likely caused by the interaction between the supernova ejecta and a dense circumstellar medium. Our results indicate that about 3 M of hydrogen-free dense circumstellar media might exist near the progenitors of SN 2007bi and SN 1999as. These massive circumstellar media would significantly affect the light curve and spectral properties of the supernovae. The precursors that are sometimes observed in superluminous supernovae might be related to the collision of the ejecta with such dense circumstellar media. We also confirm results of previous studies that synthetic spectra from pair-instability supernova models do not match the early spectra of SN 2007bi and SN 1999as.